Margaret I (Danish: Margrete Valdemarsdatter, Norwegian: Margrete Valdemarsdatter, Swedish: Margareta Valdemarsdotter, Icelandic: Margrét Valdimarsdóttir; born in 1353 at the Søborg Castle, died on 28 October 1412 in Flensburg), was ruler and for a brief time monarch of Denmark, Norway and Sweden, though there are ambiguities regarding her specific titles. She was the founder of the Kalmar Union, which spanned Scandinavia for over a century. Margaret was known as a wise, energetic and capable leader, earning the nickname "Semiramis of the North", or "the Lady King". Though the latter was a derisive nickname invented by her rival Albert of Mecklenburg, it became widely used in recognition of her capabilities.

The youngest daughter of King Valdermar Atterdag, Margaret was more of a practical, patient administrator and diplomat, albeit one of high aspirations and a strong will, who intended to unite Scandinavia forever into one single entity with the strength to resist and compete against the might of the Hanseatic League. She did not leave any biological heirs, with the early demise of her only son, though some historians suggest she had an illegitimate daughter with Abraham Brodersson. Margaret was ultimately succeeded by a string of incompetent monarchs, despite her efforts to raise and educate her heir Eric of Pomerania and his bride Philippa of England. Philippa in particular was an excellent pupil, but died young. Ultimately, the Union into which she put so much effort and hope gradually disintegrated.

Some historians have criticized Margaret for favouring Denmark and being too autocratic, though she is generally thought to have been highly regarded in Norway and respected in Denmark and Sweden. She was painted in a negative light in contemporary religious chronicles, as she had no qualms suppressing the Church to promote royal power.

Margaret is known in Denmark as "Margrethe I" to distinguish her from the current queen, who chose to be known as Margrethe II in recognition of her predecessor.